The present invention relates generally to the inspection of components employing eddy current techniques and, more particularly, to a three-dimensional eddy current probe array which provides complete coverage of an inspection surface without mechanical scanning.
Eddy current inspection is a commonly used technique for non-destructively detecting discontinuities or flaws in the surface of various components, for example, aircraft engine parts and aircraft skin surfaces. Very briefly, eddy current inspection is based on the principle of electromagnetic induction in which a drive coil is employed to induce eddy currents within the material under inspection, and secondary magnetic fields resulting from the eddy currents are detected by a sense coil, generating signals which are subsequently processed for the purpose of detecting flaws.
Eddy current testing for flaws in conductive materials is typically done by mechanically scanning a single probe in two dimensions. For example, U.S. Pat. No. 5,345,514, entitled "Method for Inspecting Components Having Complex Geometric Shapes", describes methods for interpreting eddy current image data acquired by a single probe, particularly in the context of inspecting a high pressure turbine (HPT) disk dovetail slot.
Although effective, the single probe scanning method is time consuming. Probe arrays have been developed to improve the scanning rate, as well as to increase flaw detection sensitivity. For example, General Electric High Density Interconnect (HDI) technology has been used to fabricate flexible eddy current probe arrays. In particular, Hedengren et al., U.S. Pat. No. 5,389,876, entitled "Flexible Eddy Current Surface Measurement Array for Detecting Near Surface Flaws in a Conductor Part", the entire disclosure of which is hereby expressly incorporated by reference, discloses a hybrid method of both electronic and mechanical scanning employing an eddy current probe array comprising a plurality of spatially correlated eddy current probe elements disposed within a flexible interconnecting structure which may be employed to collect a discrete plurality of spatially correlated eddy current measurements for non-destructive near surface flaw detection. An array of such elements can, in a single uni-directional scan, accommodate inspecting an area covered by the active width of the array. Thus, the array is mechanically scanned in a direction perpendicular to a row of sense elements, and electronically scanned along the row. Such an array typically includes two staggered rows of sense elements to in effect interleave the tracks defined by the individual sense elements during the mechanical scan for more complete coverage of an inspection surface.
Suitable electronics for acquiring data from a probe array such as is disclosed in U.S. Pat. No. 5,389,876 are disclosed in Young et al. U.S. Pat. No. 5,182,513, entitled "Method and Apparatus for a Multi-Channel Multi-Frequency Data Acquisition System for Nondestructive Eddy Current Inspection Testing", which patent is expressly incorporated by reference.
HDI fabrication techniques which are advantageously employed in the fabrication of the flexible array structure of the above-incorporated U.S. Pat. No. 5,389,876, are disclosed in Eichelberger et al. application Ser. No. 07/865,786, filed Apr. 7, 1992, U.S. Pat. No. 5,452,182, entitled" Flexible High Density Interconnected Structure and Flexibly Interconnected System, the entire disclosure of which is hereby also expressly incorporated by reference, which is a continuation of application Ser. No. 07/504,769, filed Apr. 5, 1990, now abandoned.
By way of further background, as disclosed in Eichelberger et al. U.S. Pat. No. 4,783,695, and related patents and applications such as Ser. No. 07/865,786, the high density interconnect structure developed by General Electric Company has previously offered many advantages in the compact assembly of electronic systems. For example, an electronic system such as a microcomputer which incorporates between thirty and fifty chips, or even more, can be fully assembled and interconnected on a single substrate which is two inches long by two inches wide by 50 mils thick. This structure is referred to herein as an "HDI structure", and the various previously-disclosed methods for fabricating HDI structures are referred to herein as "HDI fabrication techniques".
Very briefly, in typical systems employing this high density interconnect structure, a ceramic substrate is provided, and individual cavities or one large cavity having appropriate depths at the intended locations of the various chips are prepared. Various components are placed in their desired locations within the cavities and adhered by means of a thermoplastic adhesive layer.
A multi-layer high density interconnect (HDI) overcoat structure is then built up to electrically interconnect the components into an actual functioning system. To begin the HDI overcoat structure, a polyimide dielectric film, which may be Kapton.RTM. polyimide available from E. I. du Pont de Nemours Company, about 0.0005 to 0.003 inch (12.5 to 75 microns) thick is pretreated to promote adhesion and coated on one side with ULTEM.RTM. polyetherimide resin or another thermoplastic and laminated across the top of the chips, other components and the substrate, with the ULTEM.RTM. resin serving as a thermoplastic adhesive to hold the Kapton.RTM. film in place. Exemplary lamination techniques are disclosed in Eichelberger et al. U.S. Pat. No. 4,933,042.
The actual as-placed locations of the various components and contact pads thereon are determined, typically employing optical imaging techniques. Via holes are adaptively laser drilled in the Kapton.RTM. film and ULTEM.RTM. adhesive layers in alignment with the contact pads on the electronic components in their actual as-placed positions. Exemplary laser drilling techniques are disclosed in Eichelberger et al. U.S. Pat. Nos. 4,714,516 and 4,894,115; and in Loughran et al. U.S. Pat. No. 4,764,485.
A metallization layer is deposited over the Kapton.RTM. film layer and extends into the via holes to make electrical contact to the contact pads disposed thereunder. This metallization layer may be patterned to form individual conductors during the process of depositing it, or may be deposited as a continuous layer and then patterned using photoresist and etching. The photoresist is preferably exposed using a laser which, under program control, is scanned relative to the substrate to provide an accurately aligned conductor pattern at the end of the process.
Exemplary techniques for patterning the metallization layer are disclosed in Wojnarowski et al. U.S. Pat. Nos. 4,780,177 and 4,842,677; and in Eichelberger et al. U.S. Pat. No. 4,835,704 which discloses an "Adaptive Lithography System to provide High Density Interconnect". Any misposition of the individual electronic components and their contact pads is compensated for by an adaptive laser lithography system as disclosed in U.S. Pat. No. 4,835,704.
Typical such systems, being formed on a ceramic substrate, are not flexible. However, the above-incorporated Eichelberger et al. application Ser. No. 07/865,786 discloses techniques for making at least portions of the high density interconnect structure flexible.
The eddy current inspection systems described herein up to this point employ either mechanical scanning (e.g. a single probe) or a hybrid method of both electronic and mechanical scanning (e.g. a probe array comprising two staggered rows).
As noted above, scanning a single probe in two dimensions is a time-consuming process. Accordingly, a variety of static scanning approaches have been proposed in the literature, whereby a two-dimensional array of sense elements is placed in a stationary position, and scanning is accomplished by electronically switching between the elements. Examples of this approach are disclosed in the following literature references: Bert A. Auld, "Probe-Flaw Interactions with Eddy Current Array Probes", Review of Progress in Quantitative NDE 10, edited by D. O. Thompson and D. E. Chementi (Plenum Press, New York, 1991), pages 951-955; Yehuda D. Krampfner and Duane D. Johnson, "Flexible Substrate Eddy Current Coil Arrays", Review of Progress in Quantitative NDE 7, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York, 1988), pages 471-478; and Mirek Macecek, "Advanced Eddy Current Array Defect Imaging", Review of Progress in Quantitative NDE 10, edited by D. O. Thompson and D. E. Chimenti (Plenum Press, New York 1991), pages 995-1002.
A major drawback of static scanning, recognized for example in the above-referenced Krampfner and Johnson paper, is that complete coverage of the underlying inspection area is not achieved.
Thus, the hybrid scanning approach referred to above offers an attractive compromise. One or more staggered rows of elements are mechanically scanned in a direction perpendicular to the rows, while the elements are electronically scanned along each row. Attractive trade offs are realized between electronic complexity and scanning time while, at the same time, providing the ability to selectively oversample data in one direction to enhance flaw detection capability.
Nevertheless, there are applications where, due to obstructions or other considerations, any approach involving mechanical scanning is unsuitable or undesirable.
Accordingly, there remains a need for an effective static scanning approach which provides complete coverage of the inspection surface.